1. Field of the Invention
The present invention relates to a phantom and a phantom assembly (phantom unit) used as a dummy for a human body in assessing CT dosage of x-ray radiation, and is based on Japanese Patent Application No. 2003-427312, filed on Dec. 12, 2003, the disclosure of which is incorporated herein by reference.
2. Description of Related Arts
In a conventional assessment of CT dose, phantoms according to IEC (International Electrotechnical Commission) have been used (For example, see International Electrotechnical Commission, “Evaluation and routine testing in medical imaging departments constancy tests-x-ray equipment for computed tomography,” pub. IEC, 1223-2-6 (1994); T. B. Shope, R. M. Gane, and G. C. Johnson, “A method for describing the doses delivered by transmission x-ray computed tomography,” Med. Phys. 8, 448-495 (1981); and W. Leitz, B. Axelsson, and G. Szendro, “Computed tomography does assessment-A practical Approach.” Radiat. Prot. Dosim., 57, 377-380 (1995)). In this case, for example, two phantoms, one for body portion, and another for head, are ready and each made of an acrylic resin and formed into a cylinder having a diameter of 320 mm and 160 mm, respectively, and a length of 150 mm. Cavities of 10 mm diameter were located parallel to the central axis of the cylinders, and the centers of the holes were located at the cylinder center and also 10 mm below the cylinder surface at 90 degree intervals (detailed in IEC document), and evaluation of x-ray irradiated onto a human body has been made by inserting a device for measuring dose.
In the conventional assessment of CT dose as described above, if conical beam having a large beam width of x-ray, what is called, cone beam is assessed by utilizing the phantoms, the phantoms should be form in a shape where the length of the shaft direction is large to meet the shape of the cone beam. In this case, weight and the size of the phantom are increased, inconvenient for carrying the phantom and for storing it. As a result, only an x-ray having a narrow beam width can be assessed by the conventional phantom.
When an x-ray is irradiated on such a phantom, the x-ray induces scattered radiation within the phantom, which is scattered distributed at random. Accordingly, when the assessment of the x-ray radiation (patient dose) including the scattered radiation will be made, the weight and the size of the phantom will be much more increased, leading to inconvenience in terms of portability and storage of the phantom. (For dose assessment for the cone-beam, although the weight of the phantom will be increased, the phantom length should be longer than the conventional one, because the scattered radiation is distributed wider than that in the narrow beam width.)
In order to overcome the inadequacy just mentioned, it can be considered that edges portion of phantoms are lined up in a face-to-face manner whereby the length of the phantoms in the shaft direction is set to be large as a whole. However, lining up of the phantoms has problems that fixation of the phantoms is incomplete and that x-ray radiation is passed through from gaps between the edge portions of the phantoms and, thus, no accurate assessment can be made. In order to make a long phantom from a practical point of view, we joined unit phantoms together to provide phantoms of the necessary length.
An object of the present invention is to provide a phantom and a phantom assembly which can easily be carried, in which the length of the phantom can be freely set to meet the shape of an x-ray beam, in which phantoms can be tightly connected, and which can assess CT dose of x-ray radiation in an accurate manner without passing the radial from phantoms. By inserting acrylic sticks through these holes, the cylinders were more tightly fixed to each other. Moreover, the connection portion of the phantoms was step-shaped so as not to allow direct passage of x-rays through any gaps.